Catalyst compositions based on the titanium-containing compounds supported on inorganic siliceous solid carriers for use in the epoxidation of olefins into alkylene oxides are well known in the art. Examples of such catalysts are, for instance, described in U.S. Pat. No. 4,367,342 and EP-A-345856. U.S. Pat. No. 4,367,342 discloses the use of inorganic oxygen compounds of silicon in chemical composition with at least 0.1% by weight of an oxide or hydroxide of titanium, while EP-A-345856 discloses a titania-on-silica heterogeneous catalyst which is obtainable by impregnating a silicon compound with a stream of gaseous titanium tetrachloride followed by calcinations and hydrolysis steps and optionally a silylation step.
More recently, titania-on-siliceous support type catalysts have also been described in U.S. Pat. No. 6,011,162; EP-A-734764 and Nf-A-1008686. These references also list suitable siliceous support materials including silica-containing refractory oxides such as silica-alumina and silica-magnesia, highly crystalline materials such as high silica-zeolites, silica-containing molecular sieves and amorphous silica. All these support materials have in common that they have a porous structure.
For instance, EP-A-734764 mentions silicates and silica, the latter preferably being synthetic porous silica composed of amorphous silica particles coagulated or bound to one another. Specific examples mentioned are silica gel, precipitated silica, silica powders like fumed pyrogenic silicas and several crystalline alumino-silicates. In NL-A-1008686 synthetic porous silica composed of amorphous silica particles coagulated or bound to one another is mentioned. Specific examples described are silica gel, precipitated silica, fumed pyrogenic silicas and crystalline porous silicas such as high silica content zeolites as exemplified by silicalite. Also the non-crystalline molecular sieve material MCM-41 is mentioned as a suitable material. In U.S. Pat. No. 6,011,162 synthetic porous silicas consisting of particles of amorphous silica flocculated or linked together so that they form relatively dense, close-packed masses, are described as suitable inorganic siliceous materials. Specific examples mentioned are silica gel and precipitated silica. Also mentioned are synthetic silica powders consisting of particles of amorphous silica flocculated in open-packed aggregates, which are exemplified by fumed pyrogenic silicas. Another class of suitable materials mentioned in U.S. Pat. No. 6,011,162 is the class of refractory oxides such silica-alumina, silica-magnesia, silica-zirconia and the like. Furthermore, siliceous molecular sieves like MCM-41, MCM-48 and M41S are mentioned.
However, all three references discussed in the previous paragraph eventually appoint silica gel as the preferred support material. This is, for instance, illustrated by the fact that in all working examples given in these references a silica gel is used as the carrier material.
In general, silica gels contain three-dimensional networks of aggregated silica particles of colloidal dimensions and are typically prepared by acidifying an aqueous sodium silicate solution to a pH of less than 11 by combining it with a strong mineral acid. The acidification causes the formation of monosilicilic acid (Si(OH)4), which polymerizes into particles with internal siloxane linkages and external silanol groups. At a certain pH the polymer particles aggregate, thereby forming chains and ultimately gel networks. Silicate concentration, temperature, pH and the addition of coagulants affect gelling time and final gel characteristics such as density, strength, hardness, surface area and pore volume. The resulting hydrogel is washed free of electrolytes, dried and activated. The drying procedure affects the gel characteristics. Once the dried silica gel particles are obtained their shape is fixed and their dimensions can only be reduced by temperature treatment.
Although the silica gel particles are an excellent material to be used as support material for titania-on-silica catalysts, there is still room for improvement. First of all, silica gel particles cannot be shaped into any desired form. Generally, silical gel particles are spherical, e.g. obtained by spray-drying the gel or by spraying the gel into an immiscible liquid (emulsion polymerization). Alternatively, granular gel particles are used or, if being too large, are crushed into smaller granules. If such gel particles are packed into a bed, they consequently do not have an optimum shape to minimise the pressure drop across the catalyst bed when the bed is in operation. Especially with small gel particles the pressure drop poses a problem. On the other hand, small catalyst particles are favourable as in that way the effective diffusion length can be decreased and thus more active sites can participate in the reaction, thereby resulting in reduced residence times of the feed. Such shorter residence time is favourable as it means less secondary reactions and hence an improved selectivity. In practice, therefore, a balance is sought in terms of gel particle size between a high number of active sites and an acceptable pressure drop across the catalyst bed.
A further problem with gel particles is their mechanical strength. Although the strength of these particles is acceptable, it is generally insufficient to allow re-use of spent catalyst particles. Therefore, an increased mechanical strength would be desirable.
As indicated herein before the pore structure and surface characteristics of the gel particles are determined in the gelling and drying stage. Consequently, the characteristics of a dried gel particle, i.e. the form in which these particles are normally commercially available, can no longer be modified. U.S. Pat. No. 5,808,136 describes a catalyst for making vinyl acetate monomer, which catalyst contains palladium, gold and alkali acetate as catalytically active components on a support of silicon dioxide, alumosilicate or aluminium oxide. Silica is used in the form of tablets (see Comparative Example 2 and DE-C-3803895 and DE-A-3912504). Moulding silica powder into tablets is a discontinuous, relatively complicated preparation route.
U.S. Pat. No. 6,008,389 discloses oxidation catalysts based on titanium silicate extrudates having a zeolite structure, to which support subsequently is applied from 0.01 to 30% by weight of one or more noble metals selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, platinum, rhenium, gold and silver. Contrary to the catalyst of the present invention, these catalysts are not obtained from a silica support to which subsequently titanium is chemically bonded.